Developing affirming interventions for transgender and nonbinary (TNB) therapy clients requires understanding their experiences with microaggressions in psychotherapy, yet no self-report measure of anti-TNB microaggressions in this context exists. Moreover, few studies have tested the associations between anti-TNB microaggressions and therapy processes. To better address the burden of unmet mental health care needs among TNB people, this three-study investigation designed and tested the psychometric properties of the Gender Identity and Expression Microaggressions in Therapy Scale (GIEMTS), a measure of TNB individuals' encounters with microaggressions in psychotherapy. Study 1 (N = 225) identified a four-factor model, comprising the themes of Educational Burdening, Lack of Affirmation, Inflation, and Invalidation. These subscales exhibited strong internal consistency reliabilities and demonstrated convergent and discriminant validity. The results of Study 2 (N = 435) replicated the four-factor structure through confirmatory factor analysis. However, bifactor analysis revealed that the Educational Burdening, Inflation, and Invalidation subscale scores were mostly accounted by a General Anti-TNB Microaggressions scale score-though Lack of Affirmation showed evidence of its independence. Also in Study 2, both scales were uniquely negatively associated with the working alliance. Study 3 (N = 151) found evidence for the test-retest reliability of GIEMTS scores over a 2-3-week period. Overall, the GIEMTS emerged as a robust and psychometrically sound instrument that captures the experiences of TNB individuals in therapy settings. The study concludes with valuable recommendations for training and clinical practice to bolster TNB mental health. (PsycInfo Database Record (c) 2024 APA, all rights reserved).